重载==函数

Question

I am currently working on creating an overloaded function for the == operator. I am creating an hpp file for my linked list and I can't seem to get this operator working in the hpp file.

I currently have this:

template <typename T_>
class sq_list 
{

bool operator == ( sq_list & lhs, sq_list & rhs) 
{
    return *lhs == *rhs;
};

reference operator * ()     {
        return _c;
    };

};
}

I get about 10 errors but they pretty much repeat as errors:

C2804: binary 'operator ==' has too many parameters
C2333:'sq_list::operator ==' : error in function declaration; skipping function body
C2143: syntax error : missing ';' before '*'
C4430: missing type specifier - int assumed. Note: C++ does not support default-int

I've tried changing things around but I constanly get the same errors as above

Any tips or assistance on this is greatly appreciated.

Answer 1

The member operator only has one argument, which is the other object. The first object is the instance itself:

template <typename T_>
class sq_list 
{
    bool operator == (sq_list & rhs) const // don't forget "const"!!
    {
        return *this == *rhs;  // doesn't actually work!
    }
};

This definition doesn't actually make sense, since it just calls itself recursively. Instead, it should be calling some member operation, like return this->impl == rhs.impl; .

Answer 2

You are declaring the == overload as part of the class definition, as a method instances will get. Thus, the first parameter you request, lhs , is already implicit: remember, within an instance's methods you have access to this .

class myClass {
    bool operator== (myClass& other) {
        // Returns whether this equals other
    }
}

The operator==() method as part of a class should be understood as "this object knows how to compare itself to others".

You can overload operator==() outside the class to receive two arguments, both objects being compared, if that makes more sense to you. See here: http://www.learncpp.com/cpp-tutorial/94-overloading-the-comparison-operators/

Answer 3

http://courses.cms.caltech.edu/cs11/material/cpp/donnie/cpp-ops.html

The comparison operators are very simple. Define == first, using a function signature like this:

  bool MyClass::operator==(const MyClass &other) const {
    ...  // Compare the values, and return a bool result.
  }

HOW to compare MyClass objects is all your own.